The present invention relates to the field of self protection circuitry in audio power amplifiers.
The ongoing world-wide explosion in telecommunications continues to fuel development in many areas, including audio power amplifiers. Cell phones, especially those equipped as speakerphones, depend a great deal on power amplification.
Switching power amplifiers are a common implementation in modern telecommunications devices. A switching power amplifier is analogous to a switching regulator, differing mainly in the bandwidth of the processed signals (DC in the case of the regulator, 20-20 kHz for a power amplifier). Both concepts use a strategy of turning the control device fully ON or OFF (both low dissipation conditions) in a planned way to deliver power to the load with an average value that is representative of the input signal. Besides the obvious advantage of reduced dissipation compared to their linear equivalents, they can deliver power more efficiently to the load if appropriate non-dissipative filter elements are used between the regulator/amplifier and the load.
A switching power amplifier must respond to an input signal and switch ON or OFF in response to the input. The input must be sampled and, in most implementations, the outputs are rapidly switched on and off at least twice for each sampling of the input. Theoretically, since device outputs are either completely on or completely off, they do not dissipate any power. If a device is on there is a large amount of current flowing through it, but all the voltage is across the driven load, so the power dissipated by the device is zero; and when the device is off, the voltage is large, but the current is zero so the same zero power is dissipated.
Both power amplifiers and power regulators can utilize modulators of various sorts to process the input signal into the ON/OFF drive waveforms required for low dissipation/high efficiency. Popular methods in use can include PWM (Pulse-Width Modulation) and Delta-Sigma modulation (a form of PDM or Pulse-Density-Modulation).
A problem, though, lies in the fact that present devices respond well to changes in input levels but not to fault level output changes. If a load is shorted or some other over-current condition exists, the input response switching may not respond in time to prevent damage to the amplifier or the load.
One method currently used to detect over-current faults is by way of a thermal rise detection in the amplifier chip. The limitation in this method is that there must be a current flow or the chip temperature decreases. The time delay of this protection method results in a thermal cycling of the circuit with an attendant shortening of device life if not imminent destruction.
What is needed, then, is a means for protecting an audio amplifier over-current conditions in the amplifier""s output that does not allow the device heat build-up associated with a thermal protection technique. A further need exists for a device implementing such a means to sample for the continued existence of a fault and switch on when the fault no longer exists.
The present invention pertains to a method for protecting audio amplifier circuits and their loads from shorts and other over-current conditions that avoids the device heat build-up associated with a thermal-sensing protection technique. The disclosed method provides a means for switching an amplifier off in response to a fault condition in the amplifier""s output, sampling for the fault and switching on when the fault no longer exists.
The present invention relates to a method for over-current protection of an amplifier circuit, which comprises driving an electronic load with the output of the amplifier circuit and sensing for a fault condition in the load, such as a short or over-current condition. The amplifier is switched off in response to the fault condition and can stay off for a specified time delay and the amplifier is then switched on. The fault condition is tested for again and, if cured, the amplifier can remain on until a new fault condition is detected. If the fault condition remains, testing and switching the amplifier on and off over very short time periods results in an effectively limited current in the load.